1. Field of the Invention
The invention relates to communications systems, particularly systems where a plurality of receivers and transmitters are coupled to a common bus or cable.
2. Prior Art
In some communications systems, it is desirable to couple many transmitters and receivers to a common cable or bus. For example, a single coaxial cable is routed to several buildings in a complex, with several transmitters and receivers (or transceivers) at each building coupled to the cable. Data is then transmitted from a transmitter to a selected (addressed) receiver or group of receivers. The rate at which data is transferred is limited by the bandwidth of the entire cable. A data rate of 3 megabits per second for a mile of cable is typical in these systems.
In one such communications system (Ethernet) a plurality of transceivers are permitted to transmit at any time when transmission from another transceiver is not detected. Obviously, with this system, it is possible for interference to occur between the transmissions of two transceivers. In U.S. Pat. No. 4,063,220, a circuit is described for detecting interference between the transmissions of transceivers in an Ethernet and for controlling retransmission at more random times to prevent continued interference. Other circuits associated with an Ethernet system are described in this patent.
The present invention is particularly useful in an Ethernet system although it has applications in other communications systems where a common cable is employed. The effective data rate is substantially increased with the apparatus of the present invention. In a typical arrangement with 6,000 feet of cable, an increase in data transfer of two to three, or more, is realizable.